THE SUN'S HEAT. 25 



A gaseous mass, losing heat by radiation, must, therefore, at the same 

 time, grow both smaller and hotter, until the density becomes so great 

 that the ordinary laws of gaseous expansion reach their limit, and con- 

 densation into the liquid form begins. The sun seems to have arrived 

 at this point, if, indeed, it were ever wholly gaseous, which is ques- 

 tionable. At any rate, so far as we can now make out, the exterior 

 portion i. e., the jihotosphere appears to be a shell of cloudy matter, 

 precipitated from the vapors which make up the principal mass, and 

 the progressive contraction, if it is indeed a fact, must result in a con- 

 tinual thickening of this shell and the increase of the cloud-like portion 

 of the solar mass. 



This change from the gaseous to the liquid form must also be ac- 

 companied by the liberation of an enormous quantity of heat, sufficient 

 to materially diminish the amount of contraction needed to maintain 

 the solar radiation. 



Of course, if this theory of the source of the solar heat is correct, 

 it follows that in time it must come to an end ; and, looking backward, 

 we see that there must also have been a beginning : time was when 

 there was no such solar heat as now, and the time must come when it 

 will cease. 



We do not know enough about the amount of solid and liquid 

 matter at present in the sun, or of the nature of this matter, to calcu- 

 late the future duration of the sun with great exactness, though an 

 approximate estimate can be made. The problem is a little compli- 

 cated, even on the simplest hypothesis of purely gaseous contraction, 

 because, as the sun shrinks, the force of gravity increases, and the 

 amount of contraction necessary to generate a given amount of heat 

 becomes less and less ; but this difficulty is easily met by a skillful 

 mathematician. According to Newcomb, if the sun maintains its 

 present radiation, it will have shrunk to half its present diameter in 

 about five millions of years, at the longest. As it must, when reduced 

 to this size, be eight times as dense as now, it can hardly then continue 

 to be mainly gaseous, and its temperature must have begun to fall. 

 Newcomb's conclusion, therefore, is, that it is hardly likely that the 

 sun can continue to give sufficient heat to support life on the earth 

 (such life as we now are acquainted with, at least) for ten millions of 

 years from the present time. 



It is possible to compute the past of the solar history upon this 

 hypothesis somewhat more definitely than the future. The present 

 rate of contraction being known, and the law of variation, it becomes 

 a purely mathematical problem to compute the dimensions of the sun 

 at any date in the past, supposing its heat-radiation to have remained 

 unchanged. Indeed, it is not even necessary to know anything more 

 than the present amount of radiation and the mass of the sun, to com- 

 pute how long the solar fire can have been maintained at its present 

 intensity by the process of condensation. No conclusion of geometry 



